You have to get 50 kills with the 92FS - 9mm, that is the other 9mm pistol that you find in the game other than the 9mm pistol you start with. I got this replaying chapter 20 on easy, the first 2 enemies in this chapter have this weapon so kill them and then get as many kills as you can with the pistol and repeat.
Also you should pause and save game after getting it so that the game keeps the medal points.